In an anaerobic digester, for example, waste water and/or sewage is introduced into a large tank for storage and treatment. The waste water and/or sewage contains solid material more dense than the liquid and slurry with which it is carried, and such solid material tends to migrate toward the bottom of the tank. From time to time it is desirable to mix the settled solid material and the upper liquid or slurry for efficiency of the treating process, such as bacterial breakdown of the solids. In addition, when the tank is to be emptied, a more thorough and convenient emptying can be achieved if the solids are substantially uniformly suspended in the liquid.
The problem of uniform mixing of liquids and solids has been dealt with in the past, such as in the system of Crump et al. U.S. Pat. No. 5,685,076, and Crump et al. U.S. Pat. No. 5,548,414. In the systems disclosed in those patents, submerged propeller mixers or jet nozzles are asserted to induce a “helical” flow pattern in the tank which is claimed to be effective in achieving and maintaining uniform mixing.
Other mixing apparatus for liquid-solid slurries have been proposed, such as in the systems of Strong U.S. Pat. No. 3,586,294, and German patent No. 726101. These patents appear to be concerned with creating substantial turbulence by using mixing devices inducing flow in opposite directions circumferentially of a tank.